1. Field of the Invention
The present invention relates to a disk drive using a disk as a recording medium, and more particularly to a signal processing device which is suitable for feedback control based on soft-decision values output from a soft-decision Viterbi detector incorporated in an iterative decoder, and which utilizes partial response maximum likelihood detection.
2. Description of the Related Art
Magnetic disk drives (HDDs) as typical disk drives utilizing disks as recording mediums are known. As described in, for example, U.S. Pat. Nos. 5,341,249 and 6,249,398, signal processing devices for use in recent magnetic disk drives utilize digital signal processing called “partial response maximum likelihood (PRML) detection”. In the signal processing devices described in these documents, write data is encoded into a run length limited (RLL) code. The encoded data is recorded on a disk with the timing of the recorded waveform corrected. The data recorded on the disk is read by a head. The read data or signal is amplified by a read amplifier (pre-amplifier). The amplified analog signal (read signal) is input to a variable gain amplifier. The variable gain amplifier is controlled so as to make the amplitude of the read signal constant. The read signal output from the variable gain amplifier is input to an A/D converter via an analog filter. The A/D converter converts the read signal into a quantized discrete-time sample-value sequence in synchrony with a sampling clock (read clock). This sample value sequence is equalized toward a desired response by a digital FIR (Finite Impulse Response) filter. The equalized sample value sequence is detected as a binary sequence by a Viterbi detector. The detected binary sequence is decoded by a decoder (channel code decoder) into data identical to the data written to the disk.
The signal processing devices perform feedback control for adjusting the gain of the variable gain amplifier to make the amplitude of a read signal constant. They also perform feedback control for timing adjustment (timing recovery) of a sampling clock for the A/D converter, and feedback control for adaptive control of the FIR filter. Concerning these feedback control processes, see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2001-344903, as well as the above-mentioned documents. In the feedback control disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2001-344903, a system (ideal PR (partial Response) system) equivalent to the FIR filter generates a digital value sequence of a waveform equalized in an expected response (i.e., an expected PR value sequence). More specifically, an expected PR value sequence is generated by convolution integration of a binary sequence (hard-decision values) as the output of the Viterbi detector, and PR values determined from predetermined PR parameters (7, 4, −4, −5, −2). The expected PR value sequence is used as a reference PR value sequence, and compared with the output of the FIR filter, i.e., an actual sample value sequence of a PR waveform (PR equalized waveform), thereby detecting error values for each target subjected to feedback control. On the basis of the detected error values, each target is feedback-controlled.
In recent magnetic disk drives, a signal processing technique utilizing turbo coding and iterative decoding has come to be employed to compensate for reduction of an S/N ratio (signal-to-noise ratio) caused in accordance with increases in the recording density of the disk drives. This signal processing technique is described in J. Hagenauer and P. Hoeher, “A Viterbi Algorithm with Soft-Decision Outputs and its Applications” (Proc. of IEEE Globecom, p. 1680-1689, 1989) (hereinafter referred to as “document 1”), P. Pakzad, B. Nikolic and V. Anantharam, “VLSI Architectures for Iterative Decoders in Magnetic Recording Channels” (IEEE Trans. Magn., Vol. 37, No. 2, p. 748-751, March 2001), and M. Isaka and H. Imai, “A tutorial on “parallel concatenated (Turbo) coding”, “Turbo (iterative) decoding” and related topics”, TECHNICAL REPORT OF IEICE, IT98-51, P.7-11, December 1998) (hereinafter referred to as “document 2”). The signal processing technique is also described in U.S. Pat. No. 6,108,388 (hereinafter referred to as “document 3”). Decoders for iterative decoding are called iterative decoders. An iterative decoder comprises a soft-decision Viterbi decoder (inner code decoder), de-interleaver and outer code decoder. Unlike standard Viterbi detectors, the soft-decision Viterbi detector outputs soft-decision values (outer codes) instead of a binary sequence (hard-decision values). The soft-decision values represent likelihood information that indicates the reliability of each bit forming the binary sequence. The soft-decision outputs of the soft-decision Viterbi detector are input to the outer code decoder via the de-interleaver, and are again decoded by it. The iterative decoder iterates decoding of a sample value sequence (inner codes) indicative of a PR equalized waveform, using the soft-decision Viterbi detector, and decoding of outer codes using the outer code decoder. As a result of iteration of decoding, the rate of errors that occur when data is read from a disk is reduced. The iterative decoder compares, with a threshold value, the soft-decision values obtained after the iteration of decoding, thereby outputting hard-decision values (binary sequence).
However, the iteration of decoding utilizing both the soft-decision Viterbi decoder (inner code decoder) and outer code decoder involves a data time delay. The time delay of data is a problem in a disk drive that is required to show a high throughput. To overcome this, magnetic disk drives utilizing turbo coding and iterative decoding employ an iterative decoder of a pipeline or cascade structure, as described in the documents 1 and 3. Further, documents 1 and 2 describe a Viterbi algorithm called “SOVA (Soft Output Viterbi Algorithm)” (soft-decision Viterbi detector) as a Viterbi algorithm (Viterbi detector) used in an iterative decoder. SOVA comprises only forward iterative processing, and does not need backward iterative processing performed after each forward iterative processing. Accordingly, the SOVA shows a low detection performance because of its approximate calculation, but does not require a large memory capacity and hence causes only a short data time delay.
As described above, in conventional disk drives, a PR value sequence of a reference PR waveform needed for feedback control of a predetermined control target is generated on the basis of a binary sequence (hard-decision values) as the output of a Viterbi detector. On the other hand, in disk drives utilizing iterative decoding, an iterative decoder (turbo decoder) iterates decoding (detection) of a sample value sequence of a PR equalized waveform (inner codes) utilizing an inner code decoder (soft-decision Viterbi detector), and decoding of outer codes utilizing an outer code decoder, thereby reducing the error rate.
Therefore, to perform the above-described feedback control, the disk drives utilizing iterative decoding require the Viterbi output obtained after decoding iteration to have a low error count, i.e., the output (soft-decision values) of the iterative decoder, as a Viterbi output for generating a PR value sequence of a reference PR waveform (a digital value sequence of a reference PR waveform). However, if the output of an iterative decoder is used, a large time delay occurs due to feedback control.